Preheat the oven to 180°C. Trim any fat or membrane off the chicken breasts. Place one between two large squares of clingfilm and pound with a rolling pin or similar until it is evenly flattened to about 5mm thick all over. Be careful not to split the meat by being too heavy-handed. Repeat with the second chicken breast. Now make the filling: Finely chop the spring onion to yield about 1 tablespoon. Chop the tomatoes finely. Mix the onion, tomatoes, cheese and vinegar well. Unwrap the chicken breasts, and lay upside-down. Grate over a little black pepper and then divide the filling mixture evenly between them. Wrap the sides of the chicken breast arond the stuffing to make neat parcels. Heat a griddle, brushed with a little oil and then start cooking the parcels. Begin with the top of the parcel and cook for a few minutes on each side until almost cooked through. Move to an ovenproof dish if the griddle pan cannot go in the oven, otherwise transfer it all together into the hot oven. Cook for another 5 minutes, until you are sure that the chicken is completely done. Cut carefully into the thickest part, where there is an overlap, if you need to. Remove from the oven and serve with salad or fresh vegetables and some new potatoes.